VikiLABS Working with 7-segment displays www.vikipedialabs.com July 14, 2017 Seven segment displays are made up of LEDs combined such that they can be used to display numbers and letters. As their name implies, they are made of seven LEDs plus an additional LED for a dot. Seven segment displays can be in form of common anode or common cathode. They can also be in form of single digit or multi digit seven segment displays. 1 Single digit displays f e d a g b c dp Figure 1: 7 segment display lettering 1
Common Anode (CA) displays have the anode of all LED pins connected together to 5v, therefore, a LOW voltage applied individually on each pin will light up the LED of that particular pin. Conversely, Common Cathode (CC) displays have the cathode of all LED pins connected together to ground, therefore, a HIGH voltage applied individually on a given pin will turn ON the LED of that particular pin. Fig. 2 shows the internal structure of a 1-digit 7-segment display. 3,8 GND 7 6 4 2 1 9 10 5 3,8 Vcc 7 6 4 2 1 9 10 5 (a) common cathode (b) common anode Figure 2: Internal structure of 1-digit 7 segment displays 7 segment displays normally have 8 pins which represents the each LED on the display (7 segment plus the dot making 8 LEDs) (Fig. 3a and b). However, there is also a 7 segment display called a Binary Coded Display (BCD) that has only 4 pins (Fig. 3c. In this case, the binary combination as shown in Table 1 will produce numbers 0-9 and letters A - F as shown. (a) Generic 7-segment display (b) Common anode/cathode 7-segment display Figure 3: Single-digit 7 segment displays (c) Binary Coded (BCD) 7-segment display 2
Table 1: Binary Coded Display pin configuration Pin 1 Pin 2 Pin 3 Pin 4 binary 0 0 0 0 0 0000 1 0 0 0 1 0001 2 0 0 1 0 0010 3 0 0 1 1 0011 4 0 1 0 0 0100 5 0 1 0 1 0101 6 0 1 1 0 0110 7 0 1 1 1 0111 8 1 0 0 0 1000 9 1 0 0 1 1001 A 1 0 1 0 1010 b 1 0 1 1 1011 C 1 1 0 0 1100 d 1 1 0 1 1101 E 1 1 1 0 1110 F 1 1 1 1 1111 For common anode 7-segment displays, Table 2 shows the binary and hexadecimal value that would produce numbers 0-9 and alphabets A - F, this can be traced by using Fig.??. For common cathode 7-segment displays, the binary and hexadecimal values are shown in Table 3 for numbers 0-9 and alphabets A - F. Notice that the binary values can also be produced by carrying out a NOT operation on the respective common anode binary value. 2 Multi-digit displays Multi-digit displays do not have pins equal to 8 number of digits, instead, their number of pins equals 8 + number of digits as shown in Figure 4. 7 segment displays can be connected directly to a microcontroller. 74HC573 Latch IC can likewise be used to drive 7 segment displays, the advantage of using a latch is that only 8 digital pins of a microcontroller can be used for multi-digit displays. If a latch is not used, the number of pins that is equal to the number of pins the LED display has will be 3
Table 2: Common Anode dp g f e d c b a binary hexadecimal 0 1 1 0 0 0 0 0 0 0b11000000 0xc0 1 1 1 1 1 1 0 0 1 0b11111001 0xf 9 2 1 0 1 0 0 1 0 0 0b10100100 0xa4 3 1 0 1 1 0 0 0 0 0b10110000 0xb0 4 1 0 0 1 1 0 0 1 0b10011001 0x99 5 1 0 0 1 0 0 1 0 0b10010010 0x92 6 1 0 0 0 0 0 1 0 0b10000010 0x82 7 1 1 1 1 1 0 0 0 0b11111000 0xf 8 8 1 0 0 0 0 0 0 0 0b10000000 0x80 9 1 0 0 1 0 0 0 0 0b10010000 0x90 A 1 0 0 0 1 0 0 0 0b10001000 0x88 b 1 0 0 0 0 0 1 1 0b10000011 0x83 C 1 1 0 0 0 1 1 0 0b11000110 0xc6 d 1 0 1 0 0 0 0 1 0b10100001 0xa1 E 1 0 0 0 0 1 1 0 0b10000110 0x86 F 1 0 0 0 1 1 1 0 0b10001110 0x8e OFF 1 1 1 1 1 1 1 1 0b11111111 0xf f Table 3: Common Cathode dp g f e d c b a binary hexadecimal 0 0 0 1 1 1 1 1 1 0b00111111 0x3f 1 0 0 0 0 0 1 1 0 0b00000110 0x06 2 0 1 0 1 1 0 1 1 0b01011011 0x5b 3 0 1 0 0 1 1 1 1 0b01001111 0x4f 4 0 1 1 0 0 1 1 0 0b01100110 0x66 5 0 1 1 0 1 1 0 1 0b01101101 0x6d 6 0 1 1 1 1 1 0 1 0b01111101 0x7d 7 0 0 0 0 0 1 1 1 0b00000111 0x07 8 0 1 1 1 1 1 1 1 0b01111111 0x7f 9 0 1 1 0 1 1 1 1 0b01101111 0x6f A 0 1 1 1 0 1 1 1 0b01110111 0x77 b 0 1 1 1 1 1 0 0 0b01111100 0x7c C 0 0 1 1 1 0 0 1 0b00111001 0x39 d 0 1 0 1 1 1 1 0 0b01011110 0x5e E 0 1 1 1 1 0 0 1 0b01111001 0x79 F 0 1 1 1 0 0 0 1 0b01110001 0x71 OFF 0 0 0 0 0 0 0 0 0b00000000 0x00 4
(a) 2-digit, 7-segment display (b) 4-digit, 7-segment display (c) 6-digit, 7-segment display (d) 8-digit, 7-segment display Figure 4: Multi-digit 7 segment displays 5
used by the microcontroller. 3 4-digit 7-segment displays The internal structure of a 4-digit 7-segment common cathode display is shown in Fig 5. The cathode of each digit are connected together, a LOW voltage on the cathode will light up the corresponding LED if its anode is HIGH while a HIGH voltage on the cathode will turn off the LEDs. D1 D2 D3 D4 Figure 5: Internal structure of 4-digit 7 segment display common cathode display The internal structure of a 4-digit 7-segment common anode display is shown in Fig 6. The anode of each digit are connected together, a HIGH voltage on the anode will light up the corresponding LED if its cathode is LOW while a LOW voltage on the anode will turn off the LEDs of the corresponding digit. The pinout for 1-digit and 4-digit 7-segment displays is shown in Fig. 7. For 1-digit displays (Fig. 7a), pin 3 and 8 are the power pins: in common anode, these pins should be connected to Vcc while for common cathode, they should be connected to GND. 6
D1 D2 D3 D4 Figure 6: Internal structure of 4-digit 7 segment display common anode display (a) 1-digit display (b) 4-digit display Figure 7: Pinout for 1-digit and 4-digit displays 7